An inflator for inflating an inflatable vehicle occupant restraint, such as an air bag, may comprise a pressure vessel which stores inflation fluid for inflating the restraint. Such an inflator is disclosed in U.S. Pat. No. 5,348,344. In the inflator disclosed in the '344 patent, the inflation fluid comprises a combustible mixture of gases. The inflator is actuated in response to a vehicle condition which indicates the occurrence of a vehicle collision. When the inflator is actuated, the combustible mixture of gases is ignited. The pressure inside the pressure vessel is then increased by the heat which is generated upon combustion of the combustible mixture of gases. When the increasing pressure reaches a predetermined elevated level, it ruptures a closure wall to open the pressure vessel. The inflation fluid then flows outward from the pressure vessel and into the vehicle occupant restraint.